Method of producing a super-capacitor

ABSTRACT

A method of fabricating a super-capacitor provides a substrate, and then adds an electrode and electrolyte template film, having a well for receiving the electrode, to the substrate. The method also adds a second electrolyte to the electrode and electrolyte template.

FIELD OF THE INVENTION

The invention generally relates to super-capacitors and, more particularthe invention relates to producing super-capacitors.

BACKGROUND OF THE INVENTION

Although the size of portable electronic devices continues to shrink,their energy requirements often do not comparably decrease. For example,a next-generation MEMS accelerometer may have a volume that is 10percent smaller and yet, require are only 5 percent less power than theprior generation MEMS accelerometer. In that case, more of the MEMS diemay be used for energy storage. Undesirably, this trend can limitminiaturization and applicability of such electronic devices.

The art has responded to this problem by developing chip-levelsuper-capacitors (also known as “micro super-capacitors”), which havemuch greater capacitances than conventional capacitors. Specifically,when compared to conventional capacitors and batteries, super-capacitorsgenerally have higher power densities, shorter charging and dischargingtimes, longer life cycles, and faster switching capabilities.

To enhance their energy storage capabilities, the electrodes of asuper-capacitor often are formed on the micron level using a fragilematerial, thus complicating fabrication. This and other similarcomplications can reduce super-capacitor yield.

SUMMARY OF VARIOUS EMBODIMENTS

In accordance with one embodiment of the invention, a method offabricating a super-capacitor provides a substrate, and then adds anelectrolyte template film, having a well for receiving the electrode,and an electrode, to the substrate. The method also adds a secondelectrolyte to the electrode and electrolyte template.

The electrolyte template film may be added to the substrate beforeadding the electrode to the substrate, or after adding the electrode.Moreover, the substrate may have a top surface, and the electrode mayinclude a plurality of discrete electrodes supported at least in part bythe substrate. At least two of the plurality of electrodes may be spacedapart with respect to the top surface of the substrate (e.g.,laterally). The substrate may include a plurality of layers, such as abase layer, an insulating layer, and at least one additional layer onthe base layer (e.g., a current collector layer).

The electrode may be formed from any of a variety of materials andphysical structures, such as a plurality of graphene monolayers. In thatcase, and in other cases, the second electrolyte may be in liquid form,enabling the method to apply a vacuum to the electrode to draw theliquid electrolyte into at least a portion of the electrode. To improvecharge storage, the method also may process the electrode to form aplurality of channels configured to receive electrolyte. Moreover, someembodiments add the electrode by filling the prescribed well withelectrode material.

The method may form an in-situ cap covering the electrolyte andelectrode. The in-situ cap preferably is formed using a process at oneor more temperatures that do not exceed about 100 degrees C. In otherwords, the process does not expose the electrolyte, for a non-negligibletime period, to temperatures exceeding about 100 degrees C. Otherembodiments using electrolytes that can withstand higher temperatureswithout irreversible damage can form the in-situ cap at temperatures ashigh as 200 degrees C. Some embodiments add the electrolyte templatefilm by using layer transfer techniques to secure the template film tothe substrate.

In accordance with another embodiment, a method of fabricating asuper-capacitor provides a substrate, receives an electrolyte templatefilm having a plurality of wells, and couples the electrolyte templatefilm to the substrate. The method also adds electrode material to aplurality of the wells after the electrolyte template film is coupled toor supported by the substrate. The electrode material in the wells formsa plurality of electrodes that, together with the electrolyte templatefilm, form a top surface. Next, the method adds a liquid electrolyte tothe top surface to form a composite apparatus, causes the plurality ofelectrodes to receive the liquid electrolyte, and divides the compositeapparatus into a plurality of individual dice (e.g., throughconventional dicing, cutting, etc.).

In accordance with other embodiments, a method of fabricating asuper-capacitor forms an electrolyte template film on a multi-layersubstrate. The electrolyte template film has a well. The method alsoadds electrode material to the well of the template film so that thewell circumscribes the electrode material, and adds a second electrolyteto the electrode and electrolyte template.

BRIEF DESCRIPTION OF THE DRAWINGS

Those skilled in the art should more fully appreciate advantages ofvarious embodiments of the invention from the following “Description ofIllustrative Embodiments,” discussed with reference to the drawingssummarized immediately below.

FIG. 1 schematically shows a perspective view of a micro super-capacitorthat may be configured in accordance with illustrative embodiments ofthe invention.

FIG. 2 schematically shows a cross-sectional view of the super-capacitorshown in FIG. 1 across line 2-2.

FIG. 3 shows a process of fabricating the super-capacitor of FIG. 1 inaccordance with illustrative embodiments of the invention.

FIG. 4 schematically shows a top view of an electrolyte template filmthat may be used to fabricate a super-capacitor in accordance withillustrative embodiments of the invention.

FIG. 5 schematically shows a cross-sectional view of the process of FIG.3 through step 310.

FIG. 6 schematically shows a cross-sectional view of the process of FIG.3 through step 312.

FIG. 7 schematically shows a cross-sectional view of the process of FIG.3 through step 314.

FIG. 8 schematically shows a top, horizontal cross-sectional view of analternative embodiment of the super-capacitor.

FIG. 9 schematically shows a cross-sectional view of another embodimentof a super-capacitor that can be configured in accordance withillustrative embodiments of the invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Illustrative embodiments form micro super-capacitors in a manner thatincreases their robustness and fabrication yield. To that end,illustrative embodiments may form super-capacitors having an electrolytesubstantially surrounding one or more electrodes. Fabrication processespreferably use an electrolyte configured as a template film in aseparate process, and then add that template film to a substrate. Amongother ways, the process preferably forms the electrolyte, in whatevercapacity (e.g., as a template film or not), before forming theelectrodes of the super-capacitor. In that latter case, the electrodesare formed in pre-specified locations of the already formed electrolyte(e.g., pre-formed wells). Details of illustrative embodiments arediscussed below.

FIG. 1 schematically shows a perspective view of a micro super-capacitor(hereinafter “super-capacitor 10”) configured in accordance withillustrative embodiments of the invention. FIG. 2 schematically shows across-sectional view of the super-capacitor 10 along line 2-2 of FIG. 1.Specifically, the super-capacitor 10 is a unitary chip-level devicehaving a multilayer substrate 12 supporting a cap 14 that together forman interior chamber 16. Among other things, the interior chamber 16 hasa plurality of electrodes 18 and electrolyte material(s) (generallyidentified by reference number “20” and also referred to below as “firstelectrolyte 20A,” “second electrolyte 20B,” and “electrolyte material20”)) that together form a capacitance. In other words, the electrodes18 and electrolyte material 20 cooperate to have the capacity to store aprescribed electrical charge.

The electrodes 18 may be formed from conventional materials known in theart—preferably a porous material. For example, as discussed in greaterdetail below, the electrodes 18 may be formed from graphene, which isknown to be a porous material with tortuous interior and exteriorsurfaces. Virtually every surface of the electrode 18 exposed to theelectrolyte 20 therefore may be considered part of the surface area thecapacitor plates represented in the well-known equation:

C=ϵ*(A/D)   (1),

where:

-   -   C is capacitance,    -   ϵ is a constant,    -   A is the area, and    -   D is distance.

Indeed, those skilled in the art can use other materials to form theelectrode 18, such as activated carbon, carbon aerogel, or carbonnanotubes, to name but a few. Accordingly, discussion of graphene is byexample only and not intended to limit various other embodiments of theinvention.

In a similar manner, the electrolyte 20 can be formed from any of a widevariety of other corresponding materials. For example, electrolyte 20can be formed from an aqueous salt, such as sodium chloride, or a gel,such as a polyvinyl alcohol polymer soaked in a salt. Some embodimentsmay use an ionic liquid, in which ions are in the liquid state at roomtemperature. Although not necessarily aqueous, such electrolytes areknown to be extremely conductive due to the relatively free movement oftheir ions. The inventors believe that such an electrolyte 20 shouldproduce a super-capacitor 10 with relatively high energy storagecapacity because, as known by those skilled in the art, the energystorage of the capacitor is a function of the square of the voltage.

As noted, the electrolyte 20 preferably is generally integrated withboth the internal and external surfaces of the electrodes 18. Amongother things, the internal surfaces may be formed by tortuous internalchannels and pores within the electrodes 18. The external surfacessimply may be those surfaces visible from the electrode exteriors. Theelectrolyte 20 and noted electrode surfaces thus are considered to forman interface that stores energy.

Depending upon the electrode material, electrons can flow somewhatfreely within the electrodes 18. For example, electrons can flow withingraphene. The electrolyte 20, however, ads as an insulator and thus,does not conduct the electrons from the electrodes 18. In acorresponding manner, the electrolyte 20 has ions that can migratesomewhat freely up to the interface with the electrodes 18. Likeelectrons in the electrodes 18, ions in the electrolyte 20 do notmigrate through the interface.

When subjected to an electric field, ions within the electrolyte 20migrate to align with the electric field. This causes electrons andholes in the electrodes 18 to migrate in a corresponding manner,effectively storing charge. For example, in a prescribed electric field,positive ions in the electrolyte 20 may migrate toward a first electrodesurface, and the negative ions in the electrolyte 20 may migrate towarda second electrode surface. In that case, the positive ions near thefirst electrode surface attract electrons (in the electrode) toward thatinterface, while the negative ions near the second electrode surfaceattract holes (in the electrode) for that interface. The distance of theions to the interface plus the distance of the electrons to the sameinterface (on the opposite side of the interface) represent distance “d”of Equation 1 above.

Although useful as an electrode material, graphene still does not haveoptimal conductivity properties. Accordingly, illustrative embodimentsalso form a current collector layer on or as part of the substrate 12 toprovide exterior access to the electrodes 18. Among other things, thecurrent collector layer may be formed from a highly conductive metal,such as gold, or a highly doped semiconductor, such as polysilicon.Those skilled in the art can select other materials for this purpose.Prior art methods for fabricating a super-capacitor like that of FIGS. 1and 2 suffer from a number of drawbacks. For example, the fragilestructure of certain electrode materials, such as graphene, can becomedamaged during fabrication, which can reduce surface area and causeshort-circuits. Among other ways, some such prior art processes damagethe electrodes 18 as they pattern the electrolyte 20 with a conventionalphotoresist. To avoid inadvertent short circuits caused by damagedelectrodes 18, some such prior art processes space the electrodes 18farther apart, which creates another problem—it increases the size ofthe overall device. Illustrative embodiments aim to mitigate theseproblems.

Specifically, illustrative embodiments fabricate the super-capacitor 10in a manner that increases fabrication yield and reliability without theneed to increase its footprint or profile. To that end, FIG. 3 shows aprocess of fabricating the super-capacitor 10 of FIGS. 1 and 2 inaccordance with illustrative embodiments of the invention. It should benoted that this process is substantially simplified from a longerprocess that normally would be used to form the super-capacitor 10.Accordingly, the process of forming the super-capacitor 10 has manysteps, such as testing steps or additional passivation steps, that thoseskilled in the art likely would use. In addition, some of the steps maybe performed in a different order than that shown, or at the same time.Those skilled in the art therefore can modify the process asappropriate. It also should be noted that the process of FIG. 3 is abulk process, which forms a plurality of super-capacitors 10 on the samewafer/base at the same time. Although much less efficient, those skilledin the art can apply these principles to a process that forms only onesuper-capacitor 10.

The process begins at step 300, which forms an electrically insulatingmaterial 24 on the top surface of a base 22 (beginning the formation ofthe substrate 12). For example, the base 22 may comprise a bulk siliconwafer or a silicon-on-insulator (SOI) wafer commonly used in thesemiconductor industry. When using such wafers, the insulating material24 may include an oxide, such as silicon dioxide.

The process continues to step 302, which forms the current collectors 26on the insulator layer (i.e., this step forms a current collector layerof the substrate 12). To that end, the process may deposit a metal, suchas gold, on the top surface of the insulator. After the metal hassufficiently hardened, the process then patterns the metal layer in aconventional manner to form a two-dimensional array of currentcollectors 26 across the substrate 12. Each set of current collectors 26across the face of the substrate 12 is intended for use as oneindividual super-capacitor 10.

Next, the process adds a first electrolyte 20A at step 304. Illustrativeembodiments envision at least two different ways to form this firstelectrolyte 20A. Specifically, at step 306, the process determineswhether or not the first electrolyte 20A is in the physical form of atemplate film 28. In this context, a template film 28 may be consideredto be a free-standing film of electrolyte 20 that is substantially fullycured. In other words, the template film 28 is an independent objectthat can be moved and, in this case, added to the substrate 12 throughconventional processes. In fact, the template film 28 may be formed in aprocess that is separate from that of FIG. 3. For example, anindependent vendor may form and supply the template film 28 to asemiconductor manufacturing facility for incorporation into the processof FIG. 3.

FIG. 4 schematically shows a top view of one example of an electrolytetemplate film 28 the process may use. As shown, the template film 28 hasa generally round shape following that of the wafer, and a plurality ofopenings or wells 29 to receive electrode material 20 (discussed below).In this case, each pair of closely spaced wells 29 of the templateultimately will be part of a single super-capacitor 10. The individualwells 29 that do not have a second, closely spaced well 29 (i.e., thosewells 29 are near the edges of the template film 28) may be discarded.

Accordingly, if the first electrolyte 20A is in the form of a templatefilm 28, then step 308 uses conventional processes to transfer the film28 to the substrate 12. For example, illustrative embodiments may useconventional layer transfer processes to secure or couple theelectrolyte template film 28 to the substrate 12.

Conversely, if the electrolyte 20 is not a template film 28, then theprocess moves to step 310, which deposits a layer of the firstelectrolyte 20A onto the top, exposed surface of the substrate 12. Next,this step patterns the electrolyte 20 into a substrate layer having thenoted plurality of wells 29. For example, the step may use oxygen plasmato pattern the electrolyte layer through a patterned masking layersubsequently added to the first electrolyte 20A. The masking layersubsequently should be removed before further processing. FIG. 5 showsthe apparatus at this stage of fabrication.

After adding the first electrolyte 20A, the process begins formingelectrodes 18 in the various wells 29 (step 312). As shown, theelectrodes 18 are spaced apart with respect to the top planar surface ofthe substrate 12 to which they were attached. Any of a number ofdifferent types of electrodes 18 may be used. In some embodiments, step312 may form the electrodes 18 in the wells 29 by simple physicaldeposition (e.g., sputtering, evaporation), chemical deposition (e.g.,chemical vapor deposition and electro-deposition), or solution castingwith an air dry. In illustrative embodiments, repeated solution castingof reduced graphene oxide suspension into the current collector layersform each electrode 18 as a plurality of stacked graphene monolayers,i.e., each monolayer is a single layer of graphene atoms. The inventorsbelieve that this stacking of monolayers significantly improves theamount of expected surface area in contact with the electrolyte 20.

FIG. 6 schematically shows the apparatus at this stage of fabrication.The electrodes 18 preferably are generally flush with the top of firstelectrolyte 20A (from the perspective of FIG. 6), thus effectivelyforming a single, generally planar top surface. Other embodiments,however, may not form such a planar top surface. Again, as noted above,closely spaced electrodes 18 likely will be part of the samesuper-capacitor 10. It also should be noted, however, that althoughvarious figures show only two electrodes 18 for each super-capacitor 10,various embodiments can use different numbers of electrodes 18, such asone electrode 18, three electrodes 18, or more electrodes 18.Accordingly, discussion of two electrodes 18 is for discussion purposesonly.

The process continues to step 314, which adds a second electrolyte 20BIn the apparatus, and then diffuses and encircles the electrodes 18 withthe second electrolyte 20B. FIG. 7 schematically shows a cross-sectionalview of the apparatus at this stage of fabrication. To that end,illustrative embodiments may pour or otherwise deposit a liquidelectrolyte 20 (i.e., the second electrolyte 20B) onto the exposed topsurface, and then apply a vacuum infiltration process to the apparatus,including to the electrodes 18, which draws the liquid electrolyte 20into the porous electrode material 18.

Illustrative vacuum infiltration processes preferably substantiallyuniformly distribute the second electrolyte 2013 within the porousmaterial without damaging the morphology of the electrode 18. Since theelectrolyte 20B is in liquid form, heating is not generally necessary atthis stage. The step concludes by permitting the second electrolyte 20Bto cure, effectively integrating with the first electrolyte 20A.Alternative embodiments may use different first and second electrolytes20A and 20B that are not necessarily integrated at this step, or thesame electrolyte 20. Ideally, this step causes every exposed interiorand exterior surface of each electrode 18 to directly contactselectrolyte material 20. Indeed, although real-world processingconstraints may not permit such an ideal result, illustrativeembodiments drive toward that end. Accordingly, various embodiments drawthe liquid electrolyte 20 into at least a portion of each electrode 18.Yet other embodiments may skip this vacuum process.

At this stage of the process, after it is cured, electrolyte material 20substantially fully encapsulates the electrodes 18 and part or all ofthe current collectors 26. To provide access to the electrodes 18, step316 opens contacts to the current collectors 26. To that end,illustrative embodiments remove cured. electrolyte material 20 coveringat least portion of the current collectors 26, thus providing a windowfor interconnection with exterior devices. For example, this step mayopen a rectangular window having at least one 200 micron dimension.Conventional techniques may be used to remove this portion of theelectrolyte 20, such as by using masked oxygen plasma etching.Accordingly, when added to a larger system, illustrative embodiments canuse conventional interconnection techniques, such as wire bonds, toelectrically connect with other components.

The apparatus now needs to be physically protected from the environment.Accordingly, step 318 seals each super-capacitor 10 with a packagingmaterial or other protective shell. In other words, as noted above, thecap 14 forms the noted interior chamber 16 that encapsulates theelectrolyte 20 and electrodes 18. Some embodiments form this cap 14 asan in-situ cap, while others may form the cap 14 as a stand-alone cap.Illustrative embodiments form the cap 14 with a low temperaturepackaging material, such as gel packaging, if the electrolyte 20 is notcapable of tolerating high temperatures (e.g., temperatures above 100degrees C.). Accordingly, step 318 preferably uses processes thatoperate at one or more temperatures not exceeding about 100 degrees C.

The process concludes at step 320, which separates/singulates thevarious super-capacitors 10 formed on the substrate 12. Indeed, thoseskilled in the art can use any of a wide variety of techniques forseparating the super-capacitors 10, such as conventional saw or dicingprocesses along scribe streets or prescribed regions. Other embodimentscan use a perforated base 22, or other techniques known in the art.Regardless of the technique, this step concludes with a plurality ofdie-level super-capacitors 10 ready for testing, further processing, orcommercial use.

Accordingly, the process of FIG. 3 forms the first electrolyte 20A onthe substrate 12 before adding the electrodes 18. Alternativeembodiments, however, may form the electrodes 18 before adding the firstelectrolyte 20A. For example, the electrodes 18 may be formed as islandsand the first electrolyte 20A may be subsequently added as a templatefilm 28—with the electrodes 18 within the wells 29. In either case, thefirst electrolyte 20A may or may not be in the form of a template film28.

Moreover, various embodiments apply to super-capacitors 10 having otherconfigurations, such as those having electrodes 18 that are not spacedout along the top planar surface of the base 22. Instead, some suchsuper-capacitors 10 may have stacked electrodes 18—i.e., electrodes 18in different planes that are generally parallel with the plane of thetop surface of the base 22. FIG. 9 shows one example of such anembodiment. In this case, the process of FIG. 3 can form separatepreliminary apparatuses 40 (i.e., having respective electrodes 18 andelectrolyte 20 on substrates 12), and then secure both together to forma single super-capacitor, such as that shown in FIG. 9.

Some embodiments specially configure the electrodes 18 to facilitate ionand electron movement. To that end, FIG. 8 schematically shows a top,cross-sectional view of a super-capacitor 10 having electrode channels34 for accepting more electrolytes 20 deep within the electrodes 18.This enables ions to have a shorter travel distance. Some embodimentsmay be considered to cause the electrolyte template film 28 effectivelyform branches that root or extend into the electrode 18.

Accordingly, illustrative embodiments more efficiently and effectivelyform a super-capacitor 10 that is more robust/reliable, has a higheryield, and consequently, can have improved operating characteristics.

Although the above discussion discloses various exemplary embodiments ofthe invention, it should be apparent that those skilled in the art canmake various modifications that will achieve some of the advantages ofthe invention without departing from the true scope of the invention.

1-20. (canceled)
 21. A super-capacitor, comprising: a substrate; anelectrolyte template film having a plurality of wells and coupled to thesubstrate; a plurality of electrodes disposed in the plurality of wellsof the electrolyte template film, the plurality of electrodes and theelectrolyte template film forming a top surface; and an electrolytecoupled to the top surface.
 22. The super-capacitor as defined by claim21, wherein a first electrode of the plurality of electrodes comprises aplurality of graphene monolayers.
 23. The super-capacitor as defined byclaim 21, further comprising a plurality of channels in a firstelectrode of the plurality of the electrodes, wherein the plurality ofchannels are filled with the electrolyte.
 24. The super-capacitor asdefined by claim 21, wherein the substrate includes a base layer and atleast one additional layer on the base layer.
 25. The super-capacitor asdefined by claim 21, further comprising a cap covering the electrolytetemplate film, the electrolyte, and the plurality of electrodes.
 26. Asuper-capacitor, comprising: a substrate; an electrolyte template filmcoupled to the substrate, the electrolyte template film having a wellfor receiving an electrode; the electrode circumscribed by the well ofthe electrolyte template film and supported by the substrate; and anelectrolyte coupled to the electrode and the electrolyte template film.27. The super-capacitor as defined by claim 26, wherein the electrolytetemplate film and the electrode form a substantially planar top surfaceopposite the substrate.
 28. The super-capacitor as defined by claim 26,wherein the substrate has a top planar surface, further comprising aplurality of electrodes supported by the substrate, at least two of theplurality of electrodes being spaced apart with respect to the topplanar surface of the substrate.
 29. The super-capacitor as defined byclaim 26, wherein the substrate includes a current collector layer. 30.The super-capacitor as defined by claim 26, wherein the electrode issupported by a portion of the substrate at a bottom of the well of theelectrolyte template film.
 31. The super-capacitor as defined by claim26, wherein the electrolyte comprises a liquid electrolyte, wherein theliquid electrolyte is drawn into at least a portion of the electrode.32. The super-capacitor as defined by claim 26, wherein the electrodecomprises a plurality of graphene monolayers.
 33. The super-capacitor asdefined by claim 6, further comprising a plurality of branches in theelectrolyte, the plurality of branches rooting into the electrode. 34.The super-capacitor as defined by claim 26, wherein the electrode isenclosed by the substrate, the electrolyte, and walls of the well of theelectrolyte template film.
 35. The super-capacitor as defined by claim26, further comprising an in-situ cap covering the electrolyte and theelectrode.
 36. The super-capacitor as defined by claim 35, wherein thesubstrate includes a base layer and at least one additional layer on thebase layer.
 37. A super-capacitor, comprising: an electrolyte templatefilm on a multi-layer substrate, the electrolyte template film having awell; electrode material disposed in the well of the electrolytetemplate film, the well circumscribing the electrode material; and anelectrolyte coupled to the electrode material and the electrolytetemplate film.
 38. The super-capacitor as defined by claim 37, whereinthe electrode materials is supported by a portion of the substrate at abottom of the well of the electrolyte template film.
 39. Thesuper-capacitor as defined by claim 37, wherein the electrolyte templatefilm comprises cured electrolyte material.
 40. The super-capacitor asdefined by claim 37, further comprising multiple structures each havingan electrolyte template film and an electrode, wherein the multiplestructures are stacked to form a stacked super-capacitor.